Magnetic nanofluids and magnetic composite fluids in ...DOI: to be inserted by the publisher 25 th IAHR Symposium on Hydraulic Machinery and Systems, September 20-24, 2010, Timisoara,

© Tünde Borbáth, 2010. Published in Engineering Conferences Online (ECO): http://eco.pepublishing.com

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Magnetic nanofluids and magnetic composite fluids in rotating seal systems

Tünde Borbáth1, Doina Bica†

2, Iosif Potencz

3, Ladislau Vékás

2, István Borbáth

1, Tibor Boros

1

1Roseal Corporation

Nicolae Bălcescu street, no. 5/A, Odorheiu Secuiesc, 535600, Romania, [email protected], 2Lab. Magnetic Fluids, Center for Fundamental and Advanced Technical Research,

Romanian Academy, Timişoara Division (LMF-CFATR-RATD)

Mihai Viteazul Bvd., no. 24, Timişoara, 300223, Romania, [email protected] 3National Center for Engineering of Systems with Complex Fluids, University Politehnica Timişoara,

Mihai Viteazul Bvd., no. 1, Timişoara, 300222, Romania

Abstract

Recent results are presented concerning the development of magnetofluidic leakage-free rotating seals for vacuum

and high pressure gases, evidencing significant advantages compared to mechanical seals.

The micro-pilot scale production of various types of magnetizable sealing fluids is shortly reviewed, in particular

the main steps of the chemical synthesis of magnetic nanofluids and magnetic composite fluids with light hydrocarbon,

mineral oil and synthetic oil carrier liquids. The behavior of different types of magnetizable fluids in the rotating sealing

systems is analyzed. Design concepts, some constructive details and testing procedures of magnetofluidic rotating seals

are presented such as the testing equipment.

The main characteristics of several magnetofluidic sealing systems and their applications will be presented: vacuum

deposition systems and liquefied gas pumps applications, mechanical and magnetic nanofluid combined seals, gas valves

up to 40 bar equipped by rotating seal with magnetic nanofluids and magnetic composite fluids.

Keywords: Rotating Seal, Magnetic Nanofluids, Magnetic Composite Fluids, Gas Valves, Testing Procedures, Magnetofluidic

Applications

1. Introduction

The properties of the magnetic nanoparticles, the magnetic component of magnetic nanofluids, may be tailored by varying

their size and adapting their surface coating in order to meet the requirements of colloidal stability of magnetic nanofluids with

non-polar and polar carrier liquids. Long-term stability of concentrated magnetic nanofluids in strong magnetic fields, e.g. in

rotating seals or bearings [1, 2] imposes severe requirements on dispersion/stabilization of magnetic nanoparticles in various

organic carriers.

Magnetic composite fluids [3] develop higher yield stress as conventional magnetorheological (MR) fluids [4], therefore these

nano-micro structured magnetizable fluids to be considered here are envisaged beside high pressure magnetic seals, also for MR

clutches and brakes [5] capable of transferring controllable high torques with a fast response time in special hydraulic turbine

designs, without introducing noise and vibrations. Some of these MR brakes exploit also the magnetic sealing capabilities of MR

fluids [6].

Mechanical-magnetofluidic seals [7] were developed for liquid sealing applications for hydraulic equipments, e.g. for special

pumps or taps.

2. Synthesis of magnetic fluids

Details of the multi-step procedures developed at the Laboratory of magnetic fluids from Timisoara and applied on micropilot

scale at the ROSEAL Co. from Odorheiu Secuiesc are given in [8, 9, 3]. These basic procedures were refined and optimized for

synthesis of magnetic fluids for a given application, like rotating seals.

DOI: to be inserted by the publisher

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IAHR Symposium on Hydraulic Machinery and Systems, September 20-24, 2010, Timisoara, Romania 2/8

Long-term colloidal stability of magnetic nanofluids in rotating MF seals, especially at high volume fraction of magnetic

nanoparticles, is a complex issue connected to the synthesis procedure followed, including the nature of surfactant(s) and carrier

liquid used [8, 9, 10]. The dimensionless coupling parameter λ, which is half the ratio of the dipolar energy of two aligned dipoles

at close contact to the thermal energy, should be kept below 1 to ensure highly stable magnetic fluids. During preparation repulsive

forces due to coating of magnetic cores are introduced to prevent irreversible aggregation of particles produced by attractive van

der Waals and dipolar interactions. When the dipolar interactions are much stronger than the thermal energies, particle chains start

growing and forming more complex structures, depending on the particle volume fraction, size distribution, temperature and

magnetic field applied.

An interesting feature of magnetic nanofluid synthesis is that the relative strengths and ranges of various interaction potentials

can be controlled by the diameter of magnetic cores and the thickness of the stabilizing layer [11]. Magnetic fluids for sealing

applications [12, 7] have to be tailored in such a way to ensure high magnetization, low viscosity, low or very low vapor pressure

and excellent colloidal stability in intense and strongly non-uniform magnetic field. Usually, magnetic fluids in a sealing stage

have to withstand an intense and strongly non-uniform magnetic field, Hmax ~ 106 A/m and |grad H| ~ 10

9 A/m

2. These

requirements are sometimes difficult to fulfill simultaneously and impose special conditions on the stabilization procedure applied

in MF preparation, to avoid irreversible magnetic field induced structural processes.

The basic procedure for chemical synthesis of magnetite nanoparticles, mostly used for magnetic fluid preparation, has the

following steps: co-precipitation (at t ≈ 80oC) of magnetite from aqueous solutions of Fe

3+ and Fe

2+ ions in the presence of

concentrated NH4OH solution (25%) → subdomain magnetite particles → sterical stabilization (chemisorbtion of lauric acid (LA),

myristic acid (MA) or oleic acid (OA); 80-82oC) → phase separation → magnetic decantation and repeated washing → monolayer

covered magnetite nanoparticles + free surfactant → extraction of monolayer covered magnetite nanoparticles (acetone added;

flocculation) → stabilized magnetite nanoparticles used for magnetic nanofluid preparation.

Magnetic fluids for sealing applications synthesized at micropilot scale by ROSEAL Co.:

(a) organic non-polar carriers: dispersion of LA, MA or OA monolayer coated magnetite nanoparticles in various low vapor

pressure non-polar carriers (transformer oil and various mineral oils) (Vekas et al, 2006) at t ≈ 120-130oC → magnetic decantation/

filtration → repeated flocculation and redispersion of magnetic nanoparticles (elimination of free surfactant; advanced purification

process) → non-polar magnetic nanofluid.

(b) organic polar carriers (such as diesters, mixtures of various mineral and synthetic oils (e.g., high vacuum oils, HVO):

primary magnetic fluid on light hydrocarbon carrier → repeated flocculation and redispersion of magnetic nanoparticles

(elimination of free surfactant; advanced purification) → monolayer stabilized magnetic nanoparticles → dispersion in polar

solvent (stabilization with secondary surfactant, e.g. dodecylbenzensulphonic acid (DBS) or polymers (PIBSA), physically

adsorbed to the first layer) → polar magnetic nanofluid.

The saturation magnetization Ms of these nanofluids, at very high volume concentration (hydrodynamic volume fraction ≈ 0.6)

of surfactant coated magnetite nanoparticles, attains 80-100 kA/m.

According to [12, 7]

∆p = µ0 ∫0Hmax MdH - µ0 ∫0Hmin MdH= µ0 Ms (Hmax – Hmin) = Ms (Bmax – Bmin), (1)

the sealing capacity ∆p of a sealing stage is directly proportional to the saturation magnetization Ms . Here:

µ0 – absolute permeability

Ms – magnetic saturation

Hmax – maximum magnetic field intensity measured between the pole pieces and the shaft

Hmin – minimum magnetic field intensity measured on the surface of the magnetic fluid

Bmax – maximum magnetic induction

Bmin – minimum magnetic induction

Some sealing applications, such as in compressors, special pumps, turbines and taps, require even higher saturation

magnetization of the sealing fluid. For such applications nano-micro-structured composite magnetizable fluids (CMF) were

designed [3] whose saturation magnetization is about one order of magnitude higher and attains 450-500 kA/m. These CMFs are

high concentration magnetite nanofluid based micron range iron particle suspensions, which ensure high sealing capacity of low

rotation speed magnetic seals.

The magnetic nanoparticle content significantly improves the magnetorheological behavior of the composite MF in

comparison with a commercial MR fluid having the same magnetic solid content, but only micrometer sized particles [4]. The

characteristic shear stress vs. magnetic interaction energy shows a much more pronounced increase with magnetic induction for

the nano-microstructured magnetizable fluid sample, in comparison with the approximately linear increase observed for a

conventional MR fluid. The magnetic and flow properties of CMFs make them suitable for low rotation speed and high pressure

MF seal and also for semi-active MR brake and damping applications.

3. Constructive details

The main components of a magnetic fluid seal are shown on the Fig. 1.


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Fig. 1. Magnetic fluid seal: 1– permanent magnet; 2 – Pole pieces (soft magnetic materials);

3 – Magnetic nanofluid; 4 – Shaft (ferromagnetic material); 5 – Housing (nonmagnetic material);

6 – Bearings; 7 – “O” ring; 8 – Magnetic flux

A good magnetic fluid seal design involves careful selection of the materials and precise dimensioning. It is recommended to

use low-carbon soft magnetic materials (such as OLC 10, OLC 15) for pole pieces, while for the rotating shaft is required to have

soft magnetic material with high mechanical resistance, such as 13 CN 30 ~ 35.

To avoid magnetic flux dissipation the housing should be manufactured from nonmagnetic materials. The role of the bearings

is to keep distance between the shaft and the housing and to assure high rotational accuracy.

After selecting the appropriate materials the magnetic field inside the seal must be determined. To obtain the useful magnetic

flux that keeps the magnetic fluid in the air gap, the dissipated magnetic flux must be calculated, as well (see Fig. 2.).

Фuseful = Фmagnet – Фdissipated (2)

Фdissipated = Фdissipated1 + Фdissipated2 + Фdissipated3, (3)

where:

Фdissipated1 – dissipated flux through the housing

Фdissipated2 – dissipated flux due to the bearings and pole pieces

Фdissipated3 – dissipated flux through the air between the pole pieces

Fig. 2. Magnetic flux dissipation in magnetic fluid seals

Magnetic fluid seals are generally composed by multiple stages, i.e. multiple magnetic fluid rings maintained between the

rotating shaft and stationary parts by a properly designed magnetic system [1].

The total differential pressure for n stages that a magnetic fluid seal can sustain is given by the sum of the pressure capacities

of the individual stages:

∆pmax_total = n· ∆pmax_stage (4)

Usual operating conditions, in particular rotation of sealed shaft, are related to viscous dissipation Pv at a sealing stage:

Pv = 2π R3ω2η f (δ, t, b), (5)

where η is the viscosity of the sealing fluid, R the radius and ω the angular velocity of the shaft, t the width of a sealing stage at

the tip. The viscous heating of the fluid should not exceed about 100-120ºC, in order to avoid stabilizant desorption and

accelerated carrier liquid evaporation. At high peripheral velocity v of the rotating shaft, beside heating also the influence of

centrifugal forces have to be taken into account, which reduce the sealing capacity [1]:

∆ p (v) = ∆ p (0) – (1/2) ρv2 h/R, (6)


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where ρ is the density and h is the thickness of the magnetic fluid “O” ring.

The constrains related to viscous heating and centrifugal forces do not affect many of the applications of MF seals, which

usually encounter relatively low rotating speed of the sealed shaft, up to v ~ 10 m/s.

The effectiveness of designing magnetic fluid seals was increased by developing a software program. It takes in consideration

all the necessary information about materials and dimensions and calculates the maximum sustainable differential pressure of the

magnetic fluid seal.

4. Magnetofluidic rotating seal systems

MF rotary seals offer hermetic sealing capabilities for a long lifetime, can be used at high speeds, are non-contaminating and

present optimum torque direct drive transmission. Magnetic liquid seals are engineered for a wide range of applications and

exposure but are generally limited to sealing gases, vapors and not direct pressurized liquids.

In order to avoid the practical limits with respect to temperature, differential pressure, speed, applied loads and operating

environment, the software program for magnetic fluid rotating seals design takes into account the relations (5)-(7), the geometrical

and magnetic characteristics of the magnetic circuit, as well as other material properties.

Some example of the custom engineered magnetic fluid seals designed and produced by the Roseal Co., as well as their

applications are described below.

4.1 Vacuum deposition systems and liquefied gas pumps applications

4.1.1. Magnetic fluid feed-through for high power electric switches

This kind of feedthrough was designed especially for high power electric switches that use SF6 gas to impede the

formation of electric arc at switching-off. For safety reasons the leakage of SF6 has to be avoided for the full operating period

(several years) of the switch with rotating shaft (see Fig. 3a, b.) in a pressure range between 10-6

to 7 bar. Hundreds of MF

feedthroughs of this type are in use for several years (over 5 years) without maintenance.

Fig. 3a. Sketch of magnetic fluid feedthrough for high power electric switches with SF6. Components: 1- shaft;

2- ball bearing; 3,6- “O” ring; 4- permanent magnet; 5- non-magnetic casing; 7- polar piece; 8- safety ring.

Fig. 3b. Magnetic fluid feedthrough for high power electric switches

4.1.2. MF vacuum feedthrough for crystal growth equipment

The feedthrough presented was designed for vacuum sealing applications, in particular for crystal growth equipments.

Several years experiments show maintenance free operational lifetime up to 5 years in high vacuum up to 10-6

Torr (see

Fig.4.).

Fig. 4. MF vacuum feedthrough

4.1.3. MF feedthrough for mixers

This feedthrough was designed for applications such as boron-gadolinium mixers, which operates at a rotation speed up

to 1000 rot/min in a radiation field up to 100mR/h (see Fig. 5.).


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Fig. 5. MF feedthrough 1 – bearings; 2, 6, 8 - “O” ring; 3 – pole pieces;

4 – permanent magnet; 5 – seal casing; 7 – mixer casing; 9 - shaft

4.2 Tandem magnetic fluid seal – mechanical seal arrangements

To ensure leak-proof sealing of liquefied gases in a secure way a tandem seal was designed consisting in a mechanical seal

and a magnetic fluid rotary seal, taking advantages of both types of seals, such as relatively high sealed pressure difference and

long-term leakage-free operating regime.

4.2.1 Mechanical – magnetic fluid tandem seal for high vacuum deposition systems

The combined seal was designed for high vacuum (up to 5x10-7

Torr) deposition system (see Fig. 6.). To keep a low

pressure difference on the primary MF seal, the chamber between the two seals is connected to a preliminary vacuum pump.

Fig. 6. Magnetic fluid – mechanical seal for high vacuum deposition system: 1- non-magnetic shaft;

2- magnetically soft sleeve; 3-friction seal; 4- primary magnetic fluid seal;

5- secondary magnetic fluid seal; 6- to preliminary vacuum pump.

4.2.2 Mechanical – magnetic fluid tandem seal for liquefied gas pump

Sealing liquids with MF seals can encounter serious difficulties, since the sealing capacity of the magnetic liquid sealing

stages may be compromised when another liquid contacts them due to miscibility of the two liquids and/or due to foaming

process, especially at high rotational speed. Consequently, when sealing liquids, the magnetic fluid can be diluted or even

washed out and the seal fails. To avoid these limitations, tandem seals were engineered for vertical axis pumps for liquefied

gas. While the mechanical seal retains up to 25 (40) bars, for up to 3000 rot/min rotation speed of the shaft, the leakage-free

magnetic fluid seal prevent any escape of gases from the chamber between the two seals, up to 3 bars. The accumulated gas

in the chamber is evacuated to an external recovery system or flame (see Fig. 7a, b.).


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Fig. 7a. Sketch of a mechanical- magnetic fluid combined seal for liquefied gas pump

1 - shaft; 2 - mechanical seal; 3 - magnetic fluid seal; 4 - inlet for cooling and lubrication fluid;

5 - system for escaped process fluid evacuation

Fig. 7b. Mechanical- magnetic fluid combined seal for liquefied gas pump

Other application of the magnetic fluid seal:

• Gas valves up to 40 bar equipped by a sealing system using high magnetization magnetic nanofluids or magnetic

composite fluids (see Fig. 8a, b.)

Practically the sealing capacity of such gas valves is independent of the number of open-close cycle, assuring hermetic

sealing for o long time.

Fig. 8a. (left side) Sketch of gas valves equipped by magnetic fluid seal; 1 - Body; 2 - Valve seat;

3 – Guide; 4 - Magnetic liquid seal; 5 - Bar (Valve stem); 6 - Stuffing box; 7 - Valve Handle;

8 – Screw; 9 - Spiral elastic suspension pin;

Fig. 8b. (right side) Gas valves equipped by magnetic fluid seal

5. Testing procedures of magnetofluidic rotating seals

In order to determine the functioning parameters of the magnetic fluid seals, the test stand represented in Fig. 9 was built. It

can test magnetic fluid seals with diameters up to 240 mm in a large pressure domain: [10-7

bar – 50bar] at a rotational speed up to

3000 rot/min.


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Fig. 9. Main components of the magnetic fluid seal test stand

The experimental test module is composed of a rotating shaft driven by an electric motor, while an inverter ensures the variable

rotational speed of it. The shaft suspension is ensured by a ball bearing whose lubrication and cooling is done by the lubrication

unit. The magnetic fluid seal to be experimented is mounted inside the test chamber, which is connected to the pressure module or

the vacuum module, depending on the type of the seal.

In case of fluid sealing an additional installation is added to ensure the prescribed temperature and circulation of the fluid,

required in the test chamber, an installation for fluid preparing and circulation with two heat exchangers and an electrical cooler.

The vacuum module has two main components, a preliminary vacuum pump and a high vacuum column, capable to ensure 10-5

Torr.

The pressure module is composed by a compressed nitrogen cylinder supplied with a pressure adjuster connected to the buffer

basin through an adequate flexible pipe.

In order to evaluate the characteristics of the magnetic fluid rotating seal devices, data are collected during testing, such as

temperature of seal, outlet pressure in the buffer basin, temperature and pressure or vacuum in the test chamber and rotational

speed of the shaft.

A software program was developed to ensure proper data collection and analysis (see Fig. 10.).

Fig. 10. Test chamber with measured parameters.


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6. Sealing capabilities of different types of magnetizable fluids

From all of the investigated magnetizable fluids, the composite fluids give the highest values for the maximum

sustainable/burst pressure for a single sealing stage (Fig. 11).

0

0.5

1

1.5

2

2.5

3ba

r

NFM/P/300G

NFM/P/600G

NFM/Utr/300G

NFM/Utr/600 G

NFM/Utr/1000G

NFM/Utr/1100G

D1/Utr/5620G

D3/Utr/2220G

NFM/HVO/300G

NFM/HVO/500G

NFM/DOS/465G

Fig. 11. Comparison of the burst pressure of a single sealing stage

with magnetic nanofluids and magnetic composite fluids

D1 = 5620G, D3 = 2220G

At temperatures below 0ºC there are recommended magnetic nanofluids on light hydrocarbon carrier.

7. Conclusions

Leakage-free magnetic fluid feedthroughs and mechanical-magnetic fluid tandem seals were developed for several tens of bars

sealing capacity, using high magnetization magnetic nanofluids and composite magnetic fluids. A sealing capacity of seals rising

up to 50 bars were tested using a custom designed experimental stand. The tandem seals and composite magnetic fluids are

envisaged for hydraulic equipments, as well as for semi-active brakes and vibration dampers for hydraulic turbine units.

Acknowledgements

This work was supported by the Romanian National Authority for Scientific Research, through the research projects

NanoMagneFluidSeal – contract CEEX 83/2006 and Semarogaz-contract PNII 58/2007.

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Documents

Magnetic nanofluids and magnetic composite fluids in ...DOI: to be inserted by the publisher 25 th IAHR Symposium on Hydraulic Machinery and Systems, September 20-24, 2010, Timisoara,